drm/i915: fix i9xx irq enable/disable

Those functions are used on gen4 as well and gen4 does have a non-RCS
engine, so remove the BUG_ON and flip back the logic to what it was
before the ENGINE_READ/WRITE update

v2: update the posting read as well (Chris, Ville).

Fixes: baba6e57 ("drm/i915: take a reference to uncore in the engine and use it")
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190329165018.32953-1-daniele.ceraolospurio@intel.com

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -976,20 +976,16 @@ gen5_irq_disable(struct intel_engine_cs *engine)
 static void
 i9xx_irq_enable(struct intel_engine_cs *engine)
 {
-	GEM_BUG_ON(engine->id != RCS0);
-
 	engine->i915->irq_mask &= ~engine->irq_enable_mask;
-	ENGINE_WRITE(engine, RING_IMR, engine->i915->irq_mask);
-	ENGINE_POSTING_READ(engine, RING_IMR);
+	intel_uncore_write(engine->uncore, IMR, engine->i915->irq_mask);
+	intel_uncore_posting_read_fw(engine->uncore, IMR);
 }
 
 static void
 i9xx_irq_disable(struct intel_engine_cs *engine)
 {
-	GEM_BUG_ON(engine->id != RCS0);
-
 	engine->i915->irq_mask |= engine->irq_enable_mask;
-	ENGINE_WRITE(engine, RING_IMR, engine->i915->irq_mask);
+	intel_uncore_write(engine->uncore, IMR, engine->i915->irq_mask);
 }
 
 static void